Device and method for determining the aperture angle of a joint

ABSTRACT

A device for determining the aperture angle of a joint includes a detection device for detecting positions of joint components and/or positions of structures connected to or to be connected to the joint and a computational unit for ascertaining the aperture angle of the joint based on the detected positions.

RELATED APPLICATION DATA

[0001] This application claims priority of U.S. Provisional ApplicationNo. 60/440,700, filed on Jan. 17, 2003, which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

[0002] The present invention relates to a device and a method fordetermining the aperture angle of a joint lying between two bodystructures.

BACKGROUND OF THE INVENTION

[0003] Artificial hip joints are known in which a femur implant isintroduced into a bodily femur, after a head of the bodily joint hasbeen removed. A joint cavity is introduced into the hip as a counterbearing, which, as an abutment, can accommodate a spherical or partiallyspherical head of the femur implant. Seating the implants precisely is afundamental criterion for successfully implanting a hip joint. Even thesmallest deviations can lead to excessively rapid wear and tear and,therefore, to a short service life of the hip joint implant. In theevent of a sudden stress, such as, for example, an impact, a hipimplant, which is not introduced precisely, can easily be dislocated orloosened. Therefore, it is advantageous to intra-operatively verify thatan implanted joint is correctly seated and positioned.

[0004] It is known that, while implanting a hip joint, a surgeonmanually moves a patient's leg in various directions until it isprevented from moving further by the patient's anatomy, the implantedcomponents meeting adjacent bones, or encumbrances by tissues orligaments. An experienced surgeon uses this manual movement to assessthe flexibility of the joint and, therefore, how to correctly seat theindividual components of an implanted joint. However, a surgeon can onlyvery roughly assess whether flexibility ascertained and assessed in thisway is sufficient for the patient's daily life.

[0005] A device and a method for determining the position of a componentof an implant are known from U.S. Pat. No. 6,002,859, in which a modelof the joint and the implant components are generated and a movement ofthe joint is simulated using the models.

SUMMARY OF THE INVENTION

[0006] According to one aspect, the invention relates to determining anaperture angle of a joint lying between two body structures, such as,for example, the aperture angle of a hip joint, a knee joint, an elbowjoint or other joints. It is to be appreciated that the term “apertureangle” is intended to be understood, for example, in the case of theball joint-like hip joint, both as the angle between a fixed upper legaxis and a plane or axis defined by the position of the hip, and as thespatial position or orientation of the angle. The intention is, forexample, to ascertain how far the femur is inclined forwards, backwardsor to the side, relative for example to the hip, i.e. the spatial anglewhich the joint forms is determined. The intention is also, for example,to ascertain how a joint or parts or structures thereof are rotated, forexample, about a center axis.

[0007] It is an object of the present invention to propose a device anda method for determining the aperture angle of a joint, which can beused to precisely verify whether implant components are correctlyseated.

[0008] In one embodiment, the device for determining the aperture angleof a joint, such as, for example, a natural or an artificial joint,and/or for determining relative positions of components or structures,can be used before, while and after a joint implant is implanted inorder, for example, to intra-operatively ascertain the aperture angle orthe range of motion of a joint. If necessary, the device can be used tomake modifications to the implanted joint components or the position ofthe joint component and/or to the patient's anatomy, e.g., by surgery. Adetection device can be provided. The detection device can include, forexample, a camera for detecting visible or infrared light,electromagnetic sensors (magnetic tracking), a sound sensor, or a systembased on radio, using which the position of the components forming thejoint and/or the structures connected to the joint or to be connected tothe joint can be ascertained.

[0009] Changes in the position and/or the rotational position of one ormore components or structures can be detected, such that, for example,the changes can be ascertained in six degrees of freedom. In this way,markers can be attached in a known way to the respective componentsforming the joint or to adjacent structures. For example, markers can beattached to the hip and the femur and/or to the implanted components, inorder to detect the position of the respective structures or componentsonce the corresponding elements, provided with markers, have beenregistered. Furthermore, a computational unit can be provided to whichthe positional signals ascertained by the detection device, such as, forexample, the optically detected outlines of a structure or signalsemitted for example by markers, and, therefore, the position of theelements or structures connected to these markers is supplied, in orderto ascertain the aperture angle or the spatial angle in general of ajoint or an angle formed by two structures from this information.

[0010] The device in accordance with the invention thus enables animprovement in verifying and assessing, and, therefore, in performing,the implantation of an artificial joint, such that the outcome ofimplantation can be improved. Positional values and angular values canbe automatically ascertained, in order, for example, to verify that ajoint is correctly seated using movements performed manually orautomatically, for example, using robots. Taking into accountanatomically relevant influencing parameters, such as the ligaments,soft tissues, etc., it can be ascertained whether movements, which maybe performed using the implanted joint, are sufficient or whether theimplant or the seating of the implant still has to be changed, in orderto obtain the desired range of motion of the joint. A joint can, forexample, be moved in each direction until it is no longer possible,using normal force, to open or close the joint further due to itcolliding with a body structure or a component of the joint or due toother factors, such that the range of motion of the joint can beascertained in various directions. Precise angular values can beascertained by the device in accordance with the invention, which can becompared, for example, with predetermined reference values, to verifythe correct seating or the correct functionality of a joint, withoutbeing reliant on the experience of a surgeon.

[0011] Thus, in accordance with the invention, the actual positions ofthe joint components and/or of the body structures connected to thejoint, such as, for example, the hip and the femur, can be ascertainedthree-dimensionally, even intra-operatively. In addition, their positionrelative to each other can be determined, such that the aperture angleof the joint can be precisely measured in various directions or the sixdegrees of freedom determining the spatial position and orientation canbe precisely measured.

[0012] A data output device, such as a display for outputting theascertained aperture angle of the joint or a screen, can be provided.The three-dimensional spatial position of the joint can be showntogether on the data output device with the parameters describing theposition of the joint, such as, for example, the aperture angle and thespatial position of the aperture angle.

[0013] In one embodiment, the device for determining the aperture angleof a joint can include a device for applying defined forces onto thejoint or a particular joint component in defined directions. Forexample, the device for applying defined forces can include a robot or amanually-operable device, which can display an applied force, in orderto be able to ascertain how far a joint is moved when particular forcesare applied. This can be used to obtain defined measurements, which can,for example, be compared with previously recorded reference values.

[0014] In accordance with another embodiment of the invention, a methodfor determining the aperture angle of a joint includes detecting theposition of the structures forming the joint and ascertaining theaperture angle from the detected positions. The method can, for example,be used intra-operatively, to measure the aperture angle or theflexibility in general of a joint during or after surgery, so as toprovide information to assist the surgeon's work. No surgery isnecessary to perform the method since only the position of thecomponents forming the joint and/or of the adjacent body structures arerecorded. This can be accomplished, for example, using attached markers,by detecting the outlines or using other suitable methods, and theaperture angle is calculated from the information recorded in this way.

[0015] Recordings of the natural or artificial joint and/or of bodystructures adjacent to the joint can be used to determine the apertureangle or spatial angle of a joint, wherein nuclear spin resonance (MR)methods, computer tomography (CT) methods, ultrasound methods or othersuitable methods can be used. The recorded body structures can besub-divided into individual elements, for example, using knownsegmentation or separating methods. In this way, the borders of adjacentstructures can be obtained from the recorded data, in order to have datafor calculating the aperture angle from positional data of the bodystructures or for calculating an optimum position for a joint to beimplanted. In general, bone structures are detected in an image data setin a way that is substantially dependent on how the patient ispositioned or lying at the time the image data are detected. There arerough instructions for radiology and for the patient with respect to adesired position for recording the image data set. However, the variablepositions of the individual bone structures must be virtually moved toan initial position or neutral position to be defined, to provide aprecise basis for comparison, which also enables comparison between anumber of patients.

[0016] After segmenting, three local co-ordinate systems can be defined,such as, for example, Femur Left, Femur Right and Pelvis. These can thenbe oriented with respect to each other in accordance with a fixedspecification and can thus be moved into a defined positionalrelationship. If one then wishes to visualize positional data, such as,for example, angles, length of leg or of a joint, then this neutralposition can be used as a starting position with respect to which anaperture angle or a spatial position can be defined. In this way,initial conditions can be created, which may even be reproduced andcompared with each other for various patients.

[0017] In one embodiment, an image data set can be recorded in anarbitrary position. By segmenting individual body structures, forexample, Hip, Femur Left and Femur Right can be identified as individualelements. Coordinate systems can be assigned to the individual,segmented structures, where the coordinate systems enable the structuresto be virtually aligned in the neutral position.

[0018] Reference elements, such as, for example, reflective markers, canbe attached to the joint to be implanted. In one embodiment, thereference elements can be attached to the individual components of thejoint and/or to the body structures adjacent to the joint. This enablesone to ascertain and track the spatial position of the joint or of theindividual joint components and/or the corresponding body structures,once the respective elements have been registered. As such, the spatialpositions of the individual joint components and/or the spatialpositions of the body structure adjacent to the joint are available, forexample, for ascertaining a possible range of motion of a joint.

[0019] The parameters indicative of the spatial position and theaperture angle of the joint can be visualized, such that it is possible,preferably in real time, to see what angle currently exists.

[0020] In accordance with one embodiment, data describing theflexibility of a natural or implanted joint can be measured and stored,such that at least one reference value is available, in order to be ableto verify that a newly implanted joint is corrected seated and exhibitsthe desired flexibility. An implanted joint can, for example, beverified by comparing the measured flexibility of the implanted jointwith measurement data from natural, i.e., not implanted, joints orsuccessfully implanted joints. Due to symmetry, for example, in the caseof a hip joint, comparative data of the opposite joint to the joint tobe replaced can likewise be used to obtain reference data forpositioning a joint to be implanted.

[0021] In one embodiment, defined forces can be applied to the joint indefined directions, in order to have comparative values for definedstresses on the joint, enabling an implanted joint to be verified.

[0022] In accordance with another aspect, the invention relates to acomputer program which, when it is loaded onto a computer or run on acomputer, performs at least one of the method steps described above. Inaccordance with another aspect, the invention relates to a storagemedium for a program or a computer program product comprising such aprogram.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] These and further features of the present invention will beapparent with reference to the following description and drawings,wherein:

[0024]FIG. 1 is a schematic illustration of a device for determining theaperture angle of a joint in accordance with the invention;

[0025]FIG. 2 is a flow chart illustrating a method of determining theaperture angle of a joint in accordance with the invention; and

[0026] FIGS. 3A-3D are schematic diagrams illustrating a method ofdetermining a neutral position in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0027] The invention relates to a device and method for determining anaperture angle of a joint lying between two body structures, such as,for example, the aperture angle of a hip joint, a knee joint, an elbowjoint or other joints. It is to be appreciated that the term “apertureangle” is intended to be understood, for example, in the case of theball joint-like hip joint, both as the angle between a fixed upper legaxis and a plane or axis defined by the position of the hip, and as thespatial position or orientation of the angle. The intention is, forexample, to ascertain how far the femur is inclined forwards, backwardsor to the side, relative for example to the hip, i.e. the spatial anglewhich the joint forms is determined. The intention is also, for example,to ascertain how a joint or parts or structures thereof are rotated, forexample, about a center axis.

[0028]FIG. 1 schematically shows an artificial hip joint 10 including ajoint head 12 of a femur implant 14, which is accommodated by a jointcavity 16 of the hip. Reference elements or stars 20, which includemarkers 22 are attached to the body structures connected to the jointhead 12 and the joint cavity 16, respectively, and/or to the joint head12 and/or the joint cavity 16 themselves, in order to be able todetermine the position of the joint head 12 and the joint cavity 16. Ancamera 30, such as an infra-red (IR) camera, records light reflected bythe markers 22 of the reference stars 20 and forwards the signals to acomputational unit 40.

[0029] The computational unit 40 can display the position and/or thespatial angle in the orthopaedically defined directions of abduction,adduction, flexion, extension, internal and external rotation andchanges in leg length. It is also possible to calculate the anglebetween a previously defined axis 42 through the joint head 12 and apreviously defined axis 44 through the joint cavity 16. For calculatingthe angle, it is possible, for example, to fall back on data stored in adata base 50, which includes, for example, information on the geometryof the joint head 12, the joint cavity 16, the fixing position of thereference stars 20 or other information. Furthermore, reference valuesfor a series of measurements performed can be stored in the data base50, where these values can be used for comparison with subsequentmeasurements. A robot 60 and a force measuring device 62 (both incommunication with the computational unit 40) can be employed forapplying forces in defined directions to the joint as well as measuringrespective applied forces.

[0030] A coordinate system of the femur can, for example, be used as areference system and can be defined by a plane formed by a neck axis 52(i.e., the center axis of the neck of the femur) and a shaft axis 54(i.e., the center axis of the long bone of the femur). This plane isoriented about the rotational point of the femur, parallel to thefrontal pelvic plane.

[0031] The healthy, opposite joint or leg to the joint to be implantedcan be used as a comparative reference for, for example, the range ofmotion to be enabled or for the length of the leg.

[0032] The spatial angle ascertained by the computational unit 40 can beoutputted to a display unit, such as, for example, a screen, and therenumerically outputted as a graphical representation of the joint and/orwith additional information.

[0033] With reference now to FIG. 2, a method for determining theaperture angle of a hip joint is provided. It is to be appreciated thatwhile FIG. 2 is described with reference to a hip joint, the methodologyis applicable to other joints. In step 60, a co-ordinate system of thehip can be defined by identifying a sagittal center plane and a fronthip plane in a computer tomographic (CT) representation. In step 62, aco-ordinate system of the femur is defined by identifying a rotationalcenter point and a center axis in a computer tomographic representation.It is to be appreciated that steps 60 and 62 can be performed bothsimultaneously and sequentially, in any order, i.e., step 62 can also beperformed before step 60.

[0034] In step 64, segmentation is performed, in order to be able todistinguish the individual bone structures, such as, for example, thehip, right femur and left femur, in the recordings of a body structure.

[0035] In step 66, a virtual movement of one or both femurs and the hipto a neutral or initial position is simulated in a simulation, whereinfor example the coronal plane of the femur lies parallel to the frontplane of the hip in an initial position, to define a zero position.

[0036] In step 68, markers, such as, for example, reference stars, canbe attached to the femur and the hip and/or to the joint cavity and thejoint head, on the side to be treated.

[0037] In step 70, the respective elements are then registered. In oneembodiment, the registration can be performed using a navigation systemas is described in co-owned U.S. Pat. No. 6,351,659, which isincorporated herein by reference in its entirety.

[0038] In step 72, the flexibility and/or the range of motion of apatients femur is verified.

[0039] In step 74, the maximum possible movements, for example,expressed by aperture angles of the joint in various directions, areascertained, visualized and recorded. In this way, an abduction(abducting or moving the joint backwards), adduction (guiding or movingthe joint forwards), flexion (bending) and/or extension (stretching) ofthe hip joint or of another of a patient's joints can be performed. Itis also possible to compare the movements possible with the flexibilityof the patient's still healthy joint.

[0040] Optionally, the flexibility or range of motion of a patient'sfemur can be verified by comparing them with the flexibility or range ofmotion of a healthy joint lying symmetrical to said femur.

[0041] The joint can be moved both manually and automatically, forexample using a robot, in order to apply defined forces to the joint indefined directions.

[0042] As described above, recordings of the natural or artificial jointand/or of body structures adjacent to the joint can be used to determinethe aperture angle or spatial angle of a joint, wherein nuclear spinresonance (MR) methods, computer tomography (CT) methods, ultrasoundmethods or other suitable methods can be used. The recorded bodystructures can be sub-divided into individual elements, for example,using known segmentation or separating methods. In this way, the bordersof adjacent structures can be obtained from the recorded data, in orderto have data for calculating the aperture angle from positional data ofthe body structures or for calculating an optimum position for a jointto be implanted. In general, bone structures are detected in an imagedata set in a way that is substantially dependent on how the patient ispositioned or lying at the time the image data are detected. There arerough instructions for radiology and for the patient with respect to adesired position for recording the image data set. However, the variablepositions of the individual bone structures must be virtually moved toan initial position or neutral position to be defined, to provide aprecise basis for comparison, which also enables comparison between anumber of patients.

[0043] After segmenting, three local co-ordinate systems can be defined,such as, for example, Femur Left, Femur Right and Pelvis. These can thenbe oriented with respect to each other in accordance with a fixedspecification and can thus be moved into a defined positionalrelationship. If one then wishes to visualize positional data, such as,for example, angles, length of leg or of a joint, then this neutralposition can be used as a starting position with respect to which anaperture angle or a spatial position can be defined. In this way,initial conditions can be created, which may even be reproduced andcompared with each other for various patients.

[0044] FIGS. 3A-3D illustrate the procedure described above. An imagedata set, which can be recorded in an arbitrary position of the patient,is shown schematically in FIG. 3A. By segmenting, as shown schematicallyin FIG. 3B, the individual body structures, such as, for example, Hip,Femur Left and Femur Right can be identified as individual elements. Asshown in FIG. 3C, coordinate systems can be assigned to the individual,segmented structures. The coordinate systems can enable the structuresto be virtually aligned in the neutral position shown in FIG. 3D.

[0045] Although the invention has been described using a hip joint byway of example, it is clear that the device and method in accordancewith the invention can also be used to ascertain aperture angles forother joints, for example a knee joint or an elbow joint.

[0046] Although particular embodiments of the invention have beendescribed in detail, it is understood that the invention is not limitedcorrespondingly in scope, but includes all changes, modifications andequivalents coming within the spirit and terms of the claims appendedhereto.

What is claimed is:
 1. A method for determining an aperture angle of a joint, said method comprising: detecting at least one of (i) positions of components forming the joint and (ii) positions of structures connected to or to be connected to the joint; and ascertaining the aperture angle of the joint from the detected positions.
 2. The method as set forth in claim 1, further comprising: recording at least one of (i) joint structures and (ii) structures connected to or to be connected to the joint; and using the recorded structures to determine the aperture angle.
 3. The method as set forth in claim 2, further comprising: performing a segmentation step to sub-divide the recorded structures.
 4. The method as set forth in claim 2, further comprising: attaching reference markers to at least one of (i) the joint and (ii) the structures connected to or to be connected to the joint.
 5. The method as set forth in claim 3, further comprising: registering at least one of (i) the joint and (ii) the structures connected to or to be connected to the joint.
 6. The method as set forth in claim 5, further comprising: visualizing the ascertained aperture angle.
 7. The method as set forth in claim 1, wherein ascertained aperture angles are stored in a storage unit.
 8. The method as set forth in claim 1, further comprising: determining aperture angles of a natural joint in a plurality of directions; implanting an artificial joint; determining aperture angles of the implanted joint; and comparing the determined aperture angles of the natural joint with the determined aperture angles of the implanted joint.
 9. The method as set forth in claim 1, further comprising applying defined forces in defined directions to the joint.
 10. A computer program which, when it is loaded onto a computer or run on a computer, performs the method steps as set forth in claim
 1. 11. A machine-readable storage medium having stored thereon sequences of instructions that, when executed, cause a system to perform the method as set forth in claim
 1. 12. A device for determining an aperture angle of a joint, said device comprising: a detection device for detecting at least one of (i) positions of joint components and (ii) positions of structures connected to or to be connected to the joint; and a computational unit for ascertaining the aperture angle of the joint based on the detected positions.
 13. The device as set forth in claim 12, further comprising a storage unit for storing at least one of (i) a geometric structure of the joint and (ii) reference values for determining the aperture angle.
 14. The device as set forth in claim 12, further comprising a data output device for outputting the ascertained aperture angle.
 15. The device as set forth in claim 12, further comprising a robot and a force measuring device for applying defined forces in defined directions onto the joint. 